The present invention relates to a procedure and to a device for forming a plasma.
Elementary analyses of gas or aerosol samples are currently performed by subjecting a sample gas flow to a high temperature using external energy. Generally the sample gas is mixed with a gas that easily transforms into plasma, e.g. argon, helium or nitrogen, which may also be a component of the gas mixture under analysis. When the sample gas becomes sufficiently hot, the electrons in the in atoms of the elements become excited, and the wavelength of the light quantum or photon produced when the electrons are de-excited is characteristic of each element and its electron ring. By examining the light quanta, it is possible to determine the elements and their amounts contained in the sample.
As is known, the external energy can be produced using various systems. Previously known is an induction heater, which uses magnetic flux to transfer energy into the gas to be heated. A problem with the use of magnetic flux is how to "ignite" the gas, i.e. how to achieve a sufficient degree or ionization to induce the plasma state of the gas. A small gas quantity cannot receive a sufficient amount of energy from the magnetic flux, and this leads to the need for large apparatus using a high volume of gas flow. On the other hand, if small amounts of gas are used, the magnetic field has to be generated using a very high frequency, typically a frequency of several gigahertz. Conventionally, this problem is solved by using a spark between two electrodes to "ignite" the gas. The spark is created in the area where plasma is to be developed and it is extinguished after a plasma flame has been set up. This is not an automatic system, because if the plasma decays in consequence of an external disturbance, such as a power failure, gas supply failure or the like, it has to be ignited again with a spark.
Another prior-art method is to use only a high-voltage spark to produce a plasma. In this case, a gas is ionized using an electric spark until a breakdown occurs and the gas is converted into plasma. However, the spark is not extinguished after a plasma has been generated, but the spark is used to transfer the energy required by the plasma to the gas. As the required high power is transferred by means of a spark, the spark discharge is very unstable and difficult to control, causing serious; disturbances in the analysis of sample gases.